Duplex stainless steel

ABSTRACT

A duplex stainless steel excellent in the weldability during large heat input welding and in the stress corrosion cracking resistance in a chloride environment containing corrosive associated gases has a chemical composition consisting, by mass %, of C: 0.03% or less, Si: 0.2 to 1%, Mn: 5.0% or less, P: 0.040% or less, S: 0.010% or less, sol. Al: 0.040% or less, Ni: 4 to 8%, Cr: 20 to 28%, Mo: 0.5 to 2.0%, Cu: more than 2.0% and 4.0% or less and N: 0.1 to 0.35%, and optionally contains one or more selected from among V, Ca, Mg, B and a rare earth metal(s), with the balance being Fe and impurities; wherein the duplex stainless steel satisfies the following formulas: 
       2.2Cr+7Mo+3Cu&gt;66   (1)
 
       Cr+11Mo+10Ni&lt;12(Cu+30N)   (2),
 
     wherein the symbols represent mass % of the elements in the steel.

TECHNICAL FIELD

The present invention relates to a ferrite-austenite duplex stainlesssteel excellent in stress corrosion cracking resistance, in particularto a duplex stainless steel suitable as a steel material for line pipestransporting petroleum, natural gas or the like.

BACKGROUND ART

In petroleum and natural gas produced from oil fields and gas fields,such corrosive gasses as carbon dioxide gas (CO₂) and hydrogen sulfide(H₂S) are present as associated gases. In line pipes transporting suchhighly corrosive petroleum and natural gas, stress corrosion cracking(SCC), sulfide stress cracking (SSC), and general corrosion functioningas a factor for wall thickness reduction and the like offer problems. Inparticular, stress corrosion cracking (SCC) and sulfide stress cracking(SSC) are fast in rate of progression, hence the time in which the crackpenetrates line pipes is short, and such penetration occurs locally tooffer more serious problems. Accordingly, the steel materials for suchline pipes as aforementioned are required to have excellent corrosionresistance.

As steel materials excellent in corrosion resistance, what is calledduplex stainless steels composed of ferrite-austenite phases havehitherto been used. For example, Patent Document 1 describes a duplexstainless steel containing Cu in a content of 1 to 3% and improved incorrosion resistance in chloride and sulfide environments. PatentDocument 2 describes a duplex stainless steel in which the strength,toughness and seawater resistance are improved by appropriatelyregulating the contents of Cr, Ni, Cu, Mo, N and W and by controllingthe area fraction of the ferrite phase to 40% through 70%.

CITATION LIST

[Patent Document 1] WO 96/18751

[Patent Document 2] JP2003-171743A

SUMMARY OF INVENTION Technical Problem

In the duplex stainless steel described in Patent Document 1, thedegradation of the corrosion resistance of the weld zone tends to occurduring large heat input welding. In the duplex stainless steel describedin Patent Document 2, intermetallic compounds precipitate in the weldzone during large heat input welding, and hence embrittlement anddegradation of the corrosion resistance tend to occur in the weld zone,and additionally, on the assumption of the transportation of petroleumor natural gas, insufficient is the stress corrosion cracking resistancein a chloride environment containing corrosive associated gases such ascarbon dioxide gas and hydrogen sulfide.

The present invention has been performed for the purpose of solving theaforementioned problems, and an object of the present invention is toprovide a duplex stainless steel excellent in the weldability duringlarge heat input welding and excellent in the stress corrosion crackingresistance in the chloride environment containing corrosive associatedgases.

Solution to Problem

The present inventors performed a series of various experiments anddetailed studies for the purpose of actualizing in a duplex stainlesssteel the improvement of the weldability during large heat input weldingand the improvement of the stress corrosion cracking resistance in thechloride environment. Consequently, the present inventors have obtainedthe following findings (a) to (f).

(a) The stress corrosion cracking resistance of a duplex stainless steelcan be improved by strengthening with Mo the passivation film mainlycomposed of Cr. On the other hand, for the purpose of preventing theprecipitation of intermetallic compounds during large heat inputwelding, it is necessary to regulate the contents of Cr and Mo. However,in a high-temperature chloride environment containing carbon dioxide gasand hydrogen sulfide, when the contents of Cr and Mo are reduced, it isimpossible to obtain any excellent stress corrosion cracking resistancein the vicinity of a weld zone.

(b) For the purpose of improving the stress corrosion crackingresistance while the contents of Cr and Mo are being regulated, it isonly required that the passivation film mainly composed of Cr be able tobe strengthened with an element other than Mo. In this connection, Cu isan element having a function to reduce the corrosion rate of a steelmaterial in an acidic environment. Accordingly, the inclusion of Cu inan appropriate content in addition to Cr and Mo enables to stabilize thepassivation film and to strengthen the passivation film.

FIG. 4 is a graph in which for duplex stainless steels having variouschemical compositions, used in Examples described later, the content(mass %) of “Cr” is plotted on the X-axis and the content (mass %) of“7Mo+3Cu” is plotted on the Y-axis. With the straight line of“7Mo+3Cu=−2.2Cr+66” as a boundary, the graph can be divided into theupper right section of the “determination (◯) of non-occurrence ofstress corrosion cracking” and the lower left section of the“determination (x) of occurrence of stress corrosion cracking.”

Accordingly, it can be derived that by containing Cr, Mo and Cu so as tosatisfy the relation of the following formula (1), the passivation filmcan be strengthened:

2.2Cr+7Mo+3Cu>66   (1)

wherein the symbols of elements in formula (1) respectively representthe contents (unit: mass %) of the elements in the steel.

When the content of Cu is 2% or less by mass %, no sufficient corrosionresistance is obtained. Accordingly, Cu is required to be contained in acontent exceeding 2%.

(c) When a duplex stainless steel is welded, the micro-structure in thevicinity of the weld zone is heated in a short time and then cooled in ashort time. For the purpose of preventing the precipitation of theintermetallic compound (the sigma phase) in such a micro-structure inwhich heating and cooling are conducted in a short time, it is importantto suppress the nucleation and the nuclear growth of the sigma phase.

(d) The driving force for the nucleation of the sigma phase is increasedwith the increase of the content of Ni. Accordingly, when only thesuppression of the production of the sigma phase is considered, Ni hasonly not to be contained. However, when Ni is not contained, the ratiobetween the ferrite phase and the austenite phase largely deviates from1:1, and the toughness and the corrosion resistance are degraded.Accordingly, for the purpose of suppressing the production of the sigmaphase while the degradation of the toughness and the degradation of thecorrosion resistance are being prevented, Ni is required to be containedin an appropriate content depending on the contents of Cu and N.Specifically, by containing Ni so as to satisfy the relation of thefollowing formula (2), the production of the sigma phase can besuppressed without degrading the toughness and the corrosion resistance:

Cr+11Mo+10Ni<12(Cu+30N)   (2)

wherein the symbols of elements in formula (2) respectively representthe contents (unit: mass %) of the elements in the steel.

The left hand side of formula (2) represents the driving force for theprecipitation of the sigma phase; among the components constituting theduplex stainless steel, Cr, Mo and Ni are the elements to increase thedriving force for the nucleation of the precipitation of the sigmaphase; on the basis of various tests, it has been found that the degreesof contribution of Mo and Ni are 11 times and 10 times the degree ofcontribution of Cr, respectively.

On the other hand, the right hand side of formula (2) converselyrepresents the deterrent force against the precipitation of the sigmaphase, and on the basis of various tests, it has been found that thedegree of contribution of N is 30 times the degree of contribution ofCu, and the deterrent force of Cu is 12 times the driving force of Cr.

The manifestation mechanism of the deterrent force against theprecipitation of the sigma phase due to Cu and N is as follows. Thepresence of a Cu atom or an N atom in the vicinity of each of the Niatoms present in the crystal lattice suppresses the decrease of theinterface energy in the ferrite/austenite phase interface, which is thesite of the nucleation of the sigma phase; thus, the decrease amount ofthe free energy at the time of the precipitation reaction of the sigmaphase is made small, and hence the driving force for the crystalnucleation can be made small to be associated with the aforementionedmanifestation mechanism. Additionally, Cu precipitates in the matrix asa Cu concentrated phase in an ultrafine manner, hence a large number ofnucleation sites of the sigma phase are dispersed so as to competeagainst the ferrite/austenite phase interface which is the propernucleation site, and consequently, there occurs an effect to retard thesigma phase production, otherwise fast in growth, in theferrite/austenite phase boundary.

(e) By containing an appropriate amount of Ni to satisfy the relation ofthe aforementioned formula (2), Cu atoms and N atoms can be located inthe vicinities of the Ni atoms present in the crystal lattice. In thiscase, it is possible to suppress the decrease amount of the interfaceenergy in the ferrite phase/austenite phase interface, which is thenucleation site of the sigma phase. Accordingly, it is possible toreduce the decrease amount of the free energy at the time of theprecipitation reaction of the sigma phase, and it is possible to reducethe driving force for the nucleation of the sigma phase. Consequently,it is possible to suppress the production of the sigma phase.

(f) The nuclear growth of the sigma phase can be suppressed bycontaining an appropriate amount of Cu. Specifically, the inclusion ofan appropriate amount of Cu enables the precipitation of an ultrafine Cuconcentrated phase in the matrix during large heat input welding. The Cuconcentrated phase serves as the nucleation site of the sigma phase, andhence by precipitating a large number of Cu concentrated phases in adispersed manner, the Cu concentrated phases can be made to competeagainst the ferrite phase/austenite phase interface, which is the propernucleation site. Consequently, the growth of the sigma phase in theferrite phase/austenite phase interface can be retarded.

The present invention has been perfected on the basis of theaforementioned findings, and the gist of the present invention residesin the following items (1) to (4) regarding duplex stainless steel.

(1) A duplex stainless steel that has a chemical composition consisting,by mass %, of C: 0.03% or less, Si: 0.2 to 1%, Mn: 5.0% or less, P:0.040% or less, S: 0.010% or less, sol. Al: 0.040% or less, Ni: 4 to 8%,Cr: 20 to 28%, Mo: 0.5 to 2.0%, Cu: more than 2 0% and 4.0% or less andN: 0.1 to 0.35%, with the balance being Fe and impurities; wherein theduplex stainless steel satisfies the relations of the following formulas(1) and (2):

2.2Cr+7Mo+3Cu>66   (1)

Cr+11Mo+10Ni<12(Cu+30N)   (2)

wherein the symbols of elements in formulas (1) and (2) respectivelyrepresent the contents (unit: mass %) of the elements in the steel.

(2) The duplex stainless steel according to the item (1) above, whichfurther contains, by mass %, V: 1.5% or less, in place of part of Fe.

(3) The duplex stainless steel according to the item (1) or (2) above,which further contains, by mass %, one or more selected from among Ca:0.02% or less, Mg: 0.02% or less and B: 0.02% or less, in place of partof Fe.

(4) The duplex stainless steel according to any one of the items (1) to(3) above, which further contains, by mass %, rare earth metal(s): 0.2%or less, in place of part of Fe.

Advantageous Effects of Invention

The duplex stainless steel according to the present invention isexcellent in the weldability during large heat input welding andexcellent in the stress corrosion cracking resistance in a chlorideenvironment.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating a plate material prepared by mechanicalworking, (a) being a plane view and (b) being a front view.

FIG. 2 is a view illustrating a weld joint, (a) being a plane view and(b) being a front view.

FIG. 3 is an oblique perspective view illustrating a specimen.

FIG. 4 is a graph showing the relations between the chemicalcompositions of the duplex stainless steels according to Examples,wherein the mark ◯ represents the “determination of non-occurrence ofstress corrosion cracking” and the mark x represents the “determinationof occurrence of stress corrosion cracking.”

DESCRIPTION OF EMBODIMENTS

Hereinafter, the functional effect of the chemical composition of theduplex stainless steel according to the present invention is described,together with the reasons for limiting the contents in the chemicalcomposition. In this connection, “%” related to the contents means “mass%.”

C: 0.03% or less

C is an element effective in stabilizing the austenite phase. However,when the content of C exceeds 0.03%, carbides tend to precipitate, andthe corrosion resistance is degraded. Accordingly, the content of C isset at 0.03% or less.

Si: 0.2 to 1%

Si is able to ensure the fluidity of the molten metal during welding,and hence is an element effective in preventing weld defects. For thepurpose of obtaining this effect, Si is required to be contained in acontent of 0.2% or more. On the other hand, when the content of Siexceeds 1%, intermetallic compounds (such as the sigma phase) tend to beproduced. Accordingly, the content of Si is set at 0.2 to 1%. Thecontent of Si is preferably 0.2 to 0.5%.

Mn: 5.0% or less

Mn is a component effective in improving the hot workability through thedesulfurization and deoxidation effects during melting of the duplexstainless steel. Mn also has a function to increase the solubility of N.However, when the content of Mn exceeds 5.0%, the corrosion resistanceis degraded. Accordingly, the content of Mn is set at 5.0% or less.

P: 0.040% or less

P is mixed in the steel as an impurity, and degrades the corrosionresistance and the toughness of the steel. Accordingly, the content of Pis set at 0.040% or less.

S: 0.010% or less

S is mixed in the steel as an impurity, and degrades the hot workabilityof the steel. Sulfides offer the origins of the occurrence of pittingand degrade the pitting resistance of the steel. For the purpose ofavoiding these adverse effects, the content of S is set at 0.010% orless. The content of S is preferably 0.007% or less.

sol. Al: 0.040% or less

Al is a component effective as a deoxidizer of the steel. On the otherhand, when the content of N in the steel is large, Al precipitates asAlN (aluminum nitride), and degrades the toughness and the corrosionresistance of the steel. Accordingly, the content of Al is set at 0.040%or less. The content of Al as referred to in the present invention meansthe content of acid-soluble Al (what is called sol. Al). Al is used as adeoxidizer in the duplex stainless steel according to the presentinvention, because the content of Si as a component effective deoxidizeris suppressed, and hence. However, when the duplex stainless steel isproduced by vacuum melting, it is not necessary to contain Al.

Ni: 4 to 8%

Ni is a component effective in stabilizing austenite. When the contentof Ni exceeds 8%, the resultant decrease of the amount of ferrite makesit difficult to ensure the fundamental properties of the duplexstainless steel and also facilitates the production of intermetalliccompounds (such as the sigma phase). On the other hand, when the contentof Ni is less than 4%, the amount of ferrite comes to be too large andthus the features of the duplex stainless steel are lost. The solubilityof N in ferrite is small, and hence due to the amount of ferritebecoming too large, nitrides precipitate and the corrosion resistance isdegraded. Accordingly, the content of Ni is set at 4 to 8%.

Cr: 20 to 28%

Cr is a component effective in maintaining the corrosion resistance. Forthe purpose of obtaining the SCC resistance in a chloride environment,Cr is required to be contained in a content of 20% or more. On the otherhand, when the content of Cr exceeds 28%, the precipitation ofintermetallic compounds (such as the sigma phase) comes to beremarkable, and the degradation of the hot workability and thedegradation of the weldability are caused. Accordingly, the content ofCr is set at 20 to 28%.

Mo: 0.5 to 2.0%

Mo is an element extremely effective in improving the SCC resistance.For the purpose of obtaining this effect, Mo is required to be containedin a content of 0.5% or more. On the other hand, when the content of Moexceeds 2.0%, the precipitation of intermetallic compounds is remarkablyaccelerated during large heat input welding, and the degradation of thehot workability and the degradation of the weldability are caused.Accordingly, the content of Mo is set at 0.5 to 2.0%. The content of Mois preferably 0.7 to 1.8% and more preferably 0.8 to 1.5%.

Cu: more than 2.0% and 4.0% or less

Cu is a component effective in strengthening the passivation film mainlycomposed of Cr in a chloride environment containing corrosive acidicgasses (such as carbon dioxide gas and hydrogen sulfide gas).Additionally, Cu precipitates in the matrix in an ultrafine mannerduring large heat input welding to become nucleation sites ofintermetallic compounds (the sigma phase) so as to compete against theferrite/austenite phase interface which is the proper nucleation site.Consequently, there occurs retardation of the sigma phase production,otherwise fast in growth, in the ferrite/austenite phase interface. Forthe purpose of obtaining these effects, Cu is required to be containedin a content exceeding 2.0%. On the other hand, when Cu is contained ina content exceeding 4.0%, the hot workability of the steel is impaired.Accordingly, the content of Cu is set to be more than 2.0% and 4.0% orless.

N: 0.1 to 0.35%

N is a powerful austenite former, and is effective in improving thethermal stability and the corrosion resistance of the duplex stainlesssteel. The duplex stainless steel according to the present inventioncontains Cr and Mo, which are ferrite formers, in large amounts, andhence N is required to be contained in a content of 0.1% or more for thepurpose of establishing an appropriate balance between ferrite andaustenite. On the other hand, when the content of N exceeds 0.35%, thetoughness and the corrosion resistance of the steel are degraded due tothe occurrence of blow holes as weld defects, the nitride productioncaused by the thermal effects during welding or the like. Accordingly,the content of N is set at 0.1 to 0.35%.

In addition to the aforementioned chemical composition, Cr, Mo, Ni, Cuand N are required to satisfy the following formulas (1) and (2):

2.2Cr+7Mo+3Cu>66   (1)

Cr+11Mo+10Ni<12(Cu+30N)   (2)

wherein the symbols of elements in formulas (1) and (2) respectivelyrepresent the contents (unit: mass %) of the elements in the steel.

In the duplex stainless steel according to the present invention, thecontents of Cr and Mo are regulated for the purpose of suppressing theprecipitation of the intermetallic compounds. Accordingly, for thepurpose of strengthening the passivation film mainly composed of Cr, Cuis required to be contained in an appropriate amount in addition to Mo.In this connection, when the value of “2.2Cr+7Mo+3Cu” is 66 or less, asufficient resistance against the stress corrosion cracking (SCC) in achloride environment cannot be ensured as the case may be. Accordingly,the requirement of the above presented formula (1) is specified.

When the value of “Cr+11Mo+10Ni” is equal to or larger than the value of“12(Cu+30N),” the production of the intermetallic compounds in theferrite/austenite phase boundary during large heat input welding cannotbe sufficiently suppressed as the case maybe. In consideration of thispoint, the requirement of the above presented formula (2) is specified.

The duplex stainless steel according to the present invention has theaforementioned chemical composition, and the balance is composed of Feand impurities. The impurities as referred to herein mean the componentswhich are mixed due to various factors in the production processincluding raw materials such as ores and scraps when the duplexstainless steel is industrially produced, and are tolerated within therange not adversely affecting the present invention.

The duplex stainless steel according to the present invention maycontain, in addition to the aforementioned elements, one or more of theelements selected from at least one group of the following first tothird groups.

First group: V: 1.5% or less

Second group: Ca, Mg, B: 0.02% or less

Third group: rare earth metal (REM): 0.2% or less

Hereinafter, these optional elements are described in detail.

First group: V: 1.5% or less

V may be contained if necessary. V is effective in improving thecorrosion resistance (in particular, the corrosion resistance in anacidic environment) of the duplex stainless steel. More specifically, bycontaining V in combination with Mo and Cu, the crevice corrosionresistance can be improved. However, when the content of V exceeds 1.5%,there is an adverse possibility that the amount of ferrite isexcessively increased, and the toughness and the corrosion resistanceare degraded; accordingly, the content of V is set at 1.5% or less. Forthe purpose of stably displaying the improvement effect due to V of thecorrosion resistance of the duplex stainless steel, it is preferable tocontain V in a content of 0.05% or more.

Second group: One or more selected from among Ca: 0.02% or less, Mg:0.02% or less and B: 0.02% or less

One or more selected from among Ca, Mg and B may be contained ifnecessary. Each of Ca, Mg and B has an effect to fix S (sulfur) and O(oxygen) to improve the hot workability. In the duplex stainless steelaccording to the present invention, the content of S is regulated so asto be low, and hence the hot workability can be satisfactory even whenCa, Mg or B is not contained. However, in the case where further hotworkability is demanded under severe working conditions such as the caseof the production of seamless pipes based on a skew rolling method, itis possible to further improve the hot workability of the duplexstainless steel by containing one or more selected from among Ca, Mg andB. On the other hand, when the content of each of these elements exceeds0.02%, there is an adverse possibility that the amount of nonmetallicinclusions (such as the oxides and sulfides of Ca, Mg or B) is increasedand such inclusions offer the origins of pitting and the degradation ofthe corrosion resistance occurs. Accordingly, when these elements arecontained, the content of each of these elements is set at 0.02% orless. When two selected from among Ca, Mg and B are contained, the upperlimit of the total content is 0.04%; and when three of Ca, Mg and B arecontained, the upper limit of the total content is 0.06%. For thepurpose of stably displaying the improvement effect of the hotworkability due to Ca, Mg or B, it is preferable to contain Ca, Mg and Beach alone or in total, in a content of “S(mass %)+(½)·O(mass %)” ormore.

Third group: rare earth metal (REM): 0.2% or less

REM may be contained if necessary. Similarly to Ca, Mg and B, a rareearth metal also has an effect to fix S or O to enable furtherimprovement of the hot workability of the duplex stainless steel. On theother hand, when the content of the rare earth metal exceeds 0.2%, thereis an adverse possibility that the amount of nonmetallic inclusions(such as the oxides and sulfides of the rare earth metal) is increasedand such inclusions offer the origins of pitting and the degradation ofthe corrosion resistance occurs. Accordingly, when the rare earth metalis contained, the content of the rare earth metal is set at 0.2% orless. For the purpose of stably displaying the improvement effect of thehot workability due to REM, it is preferable to contain REM in a contentof “S(mass %)+(½)·O(mass %)” or more.

REM as referred to herein is a generic name of the 17 elementsconsisting of the 15 lanthanoid elements and Y and Sc, and one or moreof these elements may be contained. The content of REM means the totalcontent of such elements.

The duplex stainless steel according to the present invention can beproduced by the production equipment and the production method used forthe usual commercial production. For example, for the melting of theduplex stainless steel, there can be used an electric furnace, an Ar—O₂mixed gas bottom blowing decarburization furnace (AOD furnace), a vacuumdecarburization furnace (VOD furnace) or the like. The molten steelobtained by melting may be cast into ingots, or may be cast intorod-like billets or the like by a continuous casting method.

EXAMPLES

The duplex stainless steels (Present Inventions: Test Nos. 1 to 11, theComparative: Test Nos. 12 to 25) having the chemical compositions shownin below-presented Table 1 were melted by using a vacuum furnace of 150kg in capacity, and cast into ingots. Next, each of the ingots washeated to 1250° C., and forged into a 40-mm thick plate material.Subsequently, each of the plate materials was again heated to 1250° C.,and rolled so as to have a thickness of 15 mm by hot rolling (theworking temperature: 1050° C. or higher); then each of the platematerials after rolling was subjected to a solid solution heat treatment(a treatment of water cooling after being maintained in a soaked mannerat 1070° C. for 30 minutes) to prepare a test steel plate.

TABLE 1 Chemical composition (mass %, the balance: Fe and impurities)Test No. C Si Mn P S Ni sol-Al N Inventions 1 0.015 0.50 1.51 0.0100.0008 4.21 0.020 0.152 2 0.015 0.50 1.50 0.015 0.0010 5.50 0.020 0.2113 0.015 0.50 1.48 0.014 0.0007 4.51 0.020 0.181 4 0.015 0.50 1.55 0.0140.0008 5.09 0.020 0.156 5 0.015 0.50 1.52 0.016 0.0011 4.08 0.020 0.1926 0.021 0.42 1.53 0.017 0.0005 5.19 0.022 0.210 7 0.017 0.51 1.52 0.0120.0004 7.82 0.013 0.305 8 0.017 0.51 1.03 0.011 0.0008 5.19 0.013 0.2159 0.015 0.50 1.03 0.014 0.0006 5.22 0.014 0.228 10 0.016 0.50 1.03 0.0150.0009 5.22 0.014 0.202 11 0.016 0.50 1.02 0.013 0.0007 5.18 0.012 0.223Comparative 12 0.016 0.49 1.52 0.011 0.0008 5.21 0.012 0.232 13 0.0160.50 1.55 0.015 0.0005 5.22 0.008 0.085* 14 0.015 0.49 4.90 0.014 0.00054.04 0.019 0.224 15 0.016 0.46 7.11* 0.014 0.0008 2.01* 0.023 0.208 160.015 0.48 5.08* 0.015 0.0009 3.52* 0.023 0.262 17 0.036* 0.68 4.940.012 0.0004 1.49* 0.027 0.238 18 0.015 0.48 1.02 0.011 0.0001 5.080.028 0.231 19 0.015 0.50 1.03 0.011 0.0005 5.02 0.032 0.302 20 0.0150.43 0.98 0.011 0.0003 5.06 0.019 0.148 21 0.015 0.49 1.03 0.016 0.00065.08 0.020 0.185 22 0.016 0.50 1.01 0.013 0.0005 5.56 0.019 0.182 230.015 0.50 1.02 0.012 0.0008 6.10 0.015 0.182 24 0.011 0.48 1.54 0.0120.0009 5.12 0.020 0.155 25 0.014 0.49 1.56 0.015 0.0008 4.98 0.015 0.164Chemical composition (mass %, the balance: Fe and impurities) Test No.Cr Mo Cu V Ca Mg B REM Inventions 1 20.3 1.98 3.41 — — — — — 2 22.1 1.952.92 0.15 — — — — 3 23.2 1.97 2.08 0.07 — — — — 4 22.9 1.05 3.15 — — — —— 5 23.9 1.96 2.20 0.06 0.0015 — — — 6 24.1 1.55 2.12 — — — — — 7 25.21.02 2.51 — — 0.0200 — — 8 24.9 1.02 3.24 — — — 0.0005 — 9 26.0 0.512.07 — — — — 0.0012 10 27.1 0.50 2.15 0.08 — — 0.0008 — 11 27.0 0.523.20 0.01 — — — 0.0010 Comparative 12 18.1* 1.94 3.22 — — — — — 13 20.21.99 2.05 — — — — — 14 20.1 1.03 3.10 — — — — — 15 22.2 0.95 2.89 — — —— — 16 23.2 0.52 3.11 — — — — — 17 24.0 0.96 2.10 — — — — — 18 24.2 0.521.90* — — — — — 19 25.1 1.05 1.15* — — — — — 20 25.1 0.51 2.10 — — — — —21 24.8 2.11* 1.21* — — — — — 22 25.1 0.11* 2.10 — — — — — 23 26.2 0.02*2.12 — — — — — 24 26.7 1.04 1.55* — — — — — 25 26.8 0.02* 2.10 — — — — —*shows out of scope of the invention.

For the purpose of evaluating the weldability of each of these teststeel plates, first prepared were plate materials of 12 mm in thickness,100 mm in width and 200 mm in length, each having on a long side aV-type groove of a groove angle of 30 degrees. FIG. 1 shows a platematerial 10 which is prepared by mechanical working. In FIG. 1, (a) is aplan view and (b) is a front view.

Next, as shown in FIG. 2, for each of the test steels, two pieces of theplate material 10 having a shape shown in FIG. 1 were prepared andarranged so as for the groove faces to butt each other; then, a weldjoint 20 was prepared by performing multi-layer welding based ontungsten inert gas (TIG) welding from the one side of each of the platematerials. FIG. 2( a) is a plan view and FIG. 2( b) is a front view ofthe weld joint 20. As the welding material 30 of each of the weld joints20, a welding material of 2 mm in outer diameter prepared from the TestNo. 1 in Table 1 was used commonly for all the test steels. The weldingwas performed under the condition of the heat input amount of 30 kJ/cm,which was particularly highly efficient for a common welding working ofstainless steel.

Next, a specimen was sampled from the back side (the first layer side ofthe weld bead) of each of the weld joints 20 obtained as describedabove. Specifically, under the conditions that the penetration bead andthe scales during welding were allowed to remain, a specimen of 2 mm inthickness, 10 mm in width and 75 mm in length was sampled. FIG. 2 showsthe region to be sampled as a specimen with a dotted line.

FIG. 3 shows an oblique perspective view of a sampled specimen 40. Inthe specimen 40 shown in FIG. 3, the upper surface is the rolled surface(the lower surface of the weld joint in FIG. 2). As shown in FIG. 3, thelongitudinal direction of the specimen 40 is a direction perpendicularto the weld line. Each of the specimens 40 was sampled in such a waythat one of the two boundary lines between the welding material 30 andthe plate material 10, on the surface (the rolled surface) of theconcerned specimen 40, was to be located in the center of the surface ofthe concerned specimen 40.

By using each of the obtained specimens, a four-point bending test wasperfonned. In the four-point bending test, a stress corresponding to theyield stress of the specimen was applied to the specimen in a NaClaqueous solution (150° C.) having a concentration of 25 mass % intowhich CO₂ at 3 MPa had been injected under pressure. The test time ofthe four-point bending test was 720 hours.

After the four-point bending test, for each of the specimens, theoccurrence/nonoccurrence of the stress corrosion cracking was examinedby visually observing the exterior appearance and also by theobservation (magnification of field of vision: 500 times) with anoptical microscope in the cross-sectional direction (the directionperpendicular to the upper surface of the specimen in FIG. 3). Theresults of the observation are shown in Table 2. In Table 2, the caseswhere no stress corrosion cracking occurred are marked with “◯” and thecases where the stress corrosion cracking occurred are marked with “x.”

TABLE 2 2.2Cr + Stress corrosion (Cr + 11Mo + Precipitation of Test No.7Mo + 3Cu cracking 10Ni) − 12(Cu + 30N) the sigma phase Inventions 168.75 ∘ −11.46 ∘ 2 71.03 ∘ −12.45 ∘ 3 71.07 ∘ −0.15 ∘ 4 67.18 ∘ −8.61 ∘5 72.9 ∘ −9.26 ∘ 6 70.23 ∘ −7.99 ∘ 7 70.11 ∘ −25.3 ∘ 8 71.64 ∘ −28.26 ∘9 66.98 ∘ −23.11 ∘ 10 69.57 ∘ −13.72 ∘ 11 72.64 ∘ −34.16 ∘ Comparative12 63.06* x −30.62 ∘ 13 64.52* x 39.09* x 14 60.73* x −46.01 ∘ 15 64.16*x −56.81 ∘ 16 64.01* x −67.52 ∘ 17 65.82* x −61.42 ∘ 18 62.58* x −25.24∘ 19 66.02 x −35.67 ∘ 20 65.09* x 2.83* x 21 72.96 x 17.69* x 22 62.29*x −8.81 ∘ 23 64.14* x −3.54 ∘ 24 70.67 x 14.94* x 25 65.4* x −7.42 ∘*shows out of scope of the invention.

In each of the weld joints (see FIG. 2), the cross-section perpendicularto the weld line and the rolled surface was mirror polished and etched,and then an image analysis of the cross-section was performed by usingan optical microscope with a magnification of field of vision of 500times. Thus, the area fraction of a trace amount of the sigma phase inHAZ (weld heat affected zone) was measured, and the case where the areafraction of the sigma phase is 1% or more was determined that theprecipitation of the sigma phase occurred. The determination results areshown in Table 2. In Table 2, the cases determined that no precipitationof the sigma phase occurred are marked with “◯” and the cases determinedthat the precipitation of the sigma phase occurred are marked with “x.”

FIG. 4 is a graph showing the relation between “7Mo (mass %)+3Cu (mass%)” and “Cr (mass %)” for the duplex stainless steels of Test Nos. 1, 4,6, 13 and 20. In this connection, as shown in Table 2, no stresscorrosion cracking occurred in the specimens prepared from the duplexstainless steels of Test Nos. 1, 4 and 6, whereas the stress corrosioncracking occurred in the specimens prepared from the duplex stainlesssteels of Test Nos. 13 and 20. Accordingly, as shown in FIG. 4, when aborder line is drawn between the “7Mo (mass %)+3Cu (mass %)” values ofthe duplex stainless steels of Test Nos. 1, 4 and 6 and the “7Mo (mass%)+3Cu (mass %)” values of the duplex stainless steels of Test Nos. 13and 20, the border line is represented by the following formula (3):

7Mo (mass %)+3Cu(mass %)=−2.2Cr(mass %)+66   (3)

From the relation shown in FIG. 4, it can be seen that in the case wherethe “7Mo+3Cu” value is larger than the “−2.2Cr+66” value, namely, thecase where the duplex stainless steel satisfies the relation of theaforementioned formula (1), it is possible to prevent the occurrence ofthe stress corrosion cracking. In other words, as shown in Tables 1 and2, no stress corrosion cracking occurred in the specimens prepared fromthe duplex stainless steels of Test Nos. 1 to 11 in which therequirements for the chemical composition specified in the presentinvention and the aforementioned relation of formula (1) were satisfied.On the other hand, the stress corrosion cracking occurred in thespecimens prepared from the duplex stainless steels of Test Nos. 12 to18, 20, 22, 23 and 25. The stress corrosion cracking occurred in theseduplex stainless steels of Test Nos. 19, 21 and 24 probably because theduplex stainless steels of Test Nos. 19, 21 and 24 satisfied therelation of formula (1), but the contents of Cu (see Table 1) in theseduplex stainless steels did not satisfy the requirement of the presentinvention.

Also, as shown in Table 2, no trace amount of the sigma phaseprecipitated in the HAZ in the weld joints prepared from the duplexstainless steels of Test Nos. 1 to 12, 14 to 19, 22, 23 and 25satisfying the aforementioned relation of formula (2). On the otherhand, a trace amount of the sigma phase precipitated in each of the weldjoints prepared from the duplex stainless steels of Test Nos. 13, 20, 21and 24 not satisfying the relation of formula (2).

As can be seen clearly from the above-described results, the duplexstainless steels satisfying the requirements of the present inventioncan suppress the precipitation of the intermetallic compounds duringlarge heat input welding, and each have an excellent stress corrosioncracking resistance in chloride environments.

INDUSTRIAL APPLICABILITY

The duplex stainless steels according to the present invention areexcellent in weldability during large heat input welding and excellentin the stress corrosion cracking resistance in chloride environments.

REFERENCE SIGNS LIST

-   10: Plate material-   20: Weld joint-   30: Welding material-   40: Specimen

1. A duplex stainless steel that has a chemical composition consisting,by mass %, of C: 0.03% or less, Si: 0.2 to 1%, Mn: 5.0% or less, P:0.040% or less, S:
 0. 010% or less, sol. Al: 0.040% or less, Ni: 4 to8%, Cr: 20 to 28%, Mo: 0.5 to 2.0%, Cu: more than 2.0% and 4.0% or lessand N: 0.1 to 0.35%, with the balance being Fe and impurities; whereinthe duplex stainless steel satisfies the relations of the followingformulas (1) and (2):2Cr+7Mo+3Cu>66   (1)Cr+11 Mo+10Ni<12(Cu+30N)   (2) wherein the symbols of elements informulas (1) and (2) respectively represent the contents (unit: mass %)of the elements in the steel.
 2. The duplex stainless steel according toclaim 1, which further contains, by mass %, V: 1.5% or less, in place ofpart of Fe.
 3. The duplex stainless steel according to claim 1, whichfurther contains, by mass %, one or more selected from among Ca: 0.02%or less, Mg: 0.02% or less and B: 0.02% or less, in place of part of Fe.4. The duplex stainless steel according to claim 1, which furthercontains, by mass %, rare earth metal(s): 0.2% or less, in place of partof Fe.